Intumescence is a process where, under the influence of heat, a solid substance transforms into a relatively rigid, expanded foam. This foam which has a substantially lower thermal conductivity than the original substance, by virtue of this expansion, is used as a means of providing fire protection. Intumescent products, in their current form, are applied to the surface of materials and structures requiring protection from fire.
The intumescent char consists of amorphous carbon, or carbon and graphite, and condensed phosphoric acids. The char itself is not combustible. The underlying theory is that if a polymeric material can be altered so that it thermally decomposes in the same manner as an intumescent, then it will be non-flammable.
Where a substance is flammable such as wood, plastics, glass reinforced plastics these materials will increase the fire load of a compartment, or allow flame to spread along the surface of the material, thus allowing the fire to spread to areas remote from the point of initiation.
There are two ways of reducing the fire risk. Firstly, flame retardant additives may be added to the material. Most flame retardant additives are expensive and they often detract from the properties of the base material.
Alternatively, a non-flammable coating can be applied to the surface. This is only satisfactory providing the fire regime is limited. If the film has no insulation value, heat will pass through the film to the flammable substrate until it out-gases or distorts, disrupting the film and exposing the substrate to the fire regime. Because intumescent coatings expand under fire regimes to provide an insulating layer, they prevent the passage of heat to the substrate. Char formation, as in wood, is known to cause substrates to be self limiting when involved in fire.
In order to provide protection from fire, intumescent fire-retarding compositions included in paints or other coatings are often applied to the surface of materials and structures requiring such protection. Intumescent fire retarding compositions act by expanding under the influence of heat, to form an expanded foam/char layer of relatively incombustible material. This incombustible layer serves as insulation and prevents ready access of oxygen to the material upon which it is applied, thus reducing or delaying the overheating and/or combustion of the material. Additionally, intumescent materials may be included in sealants such as caulk and, upon expanding to form a foam/char layer act as heat-resistant smoke seals.
Intumescent fire retarding compositions may comprise:
(a) a source of a non-volatile acid, usually ammonium polyphosphate,
(b) an organic substance, for example a carbonific polyol that can be decomposed by the liberated acid from the ammonium polyphosphate to reduce to a source of carbon, and
(c) A source of inert volatile gases to assist the formation of the foam/char is a spumescent. The source of the volatile gases can be, for example, melamine. The intumescent foam/char of a phosphorus catalysing intumescent system typically consists of amorphous carbon, or carbon and graphite and condensed phosphoric acids.
The underlying theory behind the work is to combine the functionalities of the phosphate catalysed system into a polymeric molecule that has both resinous and intumescent properties.
As the inefficient labile groups are absent and all the functionalities are in contact at the molecular scale then no transport problem exists. For example, during the reaction of a conventional phosphate catalysed intumescent, the polyol present is dehydrated by the liberated polyphosphoric acid.
Clearly a phosphate ester has to be formed as an intermediate before the carbonific can be dehydrated to carbon. This phosphate ester decomposes immediately it is formed because the reaction temperature is above the decomposition point of the ester. However, were the phosphate ester present in the original substance, then the reaction would commence at the lower decomposition point of the ester not at the decomposition temperature of the ammonium polyphosphate.
The integration of the binder, spumescent, carbonific and catalyst functionalities of a conventional intumescent into one polymeric molecule by creating stable phosphate esters of carbonific polyols, was achieved in the Intrinsically Intumescent Polymers (IIP). The predication of the behaviour of the IIP from the original urea phytic acid salts may be considered as a proof of the theory of molecular integration of intumescent functionalities.
If the behaviour of both a conventional intumescent material and an IIP based material is considered with respect to the weight losses that occur as the material is subject to a heating regime, then, the IIP formulation is 40% more efficient at producing its foam char than a conventional formulation. As the IIP activates at below 150° C., then the fire protection provided by the insulation is available earlier in the fire regime than with APP formulations.
The IIP process was designed to provide a means of cheaply synthesising partial phosphate esters without resource to the route normally used for synthesising trimester plasticisers where the starting point is phosphorus oxychloride or phosphorus trichloride. These latter processes are inappropriate to the output of highly viscous polymeric materials. The IIP partial phosphate esters are produced by a unique process of direct esterification.